Prof. Koji Fukagata Profile

Prof. Koji Fukagata

at Keio Univ

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Publications (3)

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Proceedings Article | 23 February 2018 Paper

Precise core alignment in fabrication of polymer optical waveguides using the Mosquito method for three-dimensional optical circuits

Kumi Date, Koji Fukagata, Takaaki Ishigure

Proceedings Volume 10535, 105351Y (2018) https://doi.org/10.1117/12.2289206

KEYWORDS: Cladding, Waveguides, Polymer multimode waveguides, Polymers, Optical alignment, Device simulation, Liquids, Ultraviolet radiation, Optical circuits, Optical interconnects

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Polymer optical waveguides fabricated using the Mosquito method are expected to realize high bandwidth density 3-dimensional (3-D) on-board wiring. In the Mosquito method, the waveguides are fabricated by dispensing a liquid core monomer into a liquid cladding monomer using a microdispenser. Hence, for the on-board applications particularly 3-D wiring, the core position and alignment accuracies are important to couple the waveguides with the other optical components with high efficiency. We already succeeded in fabricating graded-index core multimode polymer optical waveguides with low propagation loss using the Mosquito method. However, the positions of the formed cores tended to deviate from the original design, since both the core and cladding monomers are in the liquid state during the Mosquito process.

In this paper, we apply a fluid analysis simulation using a COMSOL Multiphysics® in order to theoretically simulate the influence of several fabrication parameters on the core position. The calculated core height deviation from the designed height is dependent on the needle-tip height, because the core positions are influenced by the pressure distribution of cladding monomer caused by the monomer flow. Meanwhile, we find that the monomer wetting on the needle outer wall also affects the core height. When the effect of monomer wetting is taken into account, the simulated core heights are different from the results without the effect of monomer wetting and we can theoretically predict the height of the formed core. Finally, we confirm that the core height can be controlled by adjusting the needle-tip height setting in which the effect of the monomer flow and wetting theoretically calculated is taken into account in the Mosquito method.

Proceedings Article | 1 March 2016 Paper

Low-loss polymer optical waveguides with graded-index perfect circular cores for on-board interconnection

Yuki Saito, Koji Fukagata, Takaaki Ishigure

Proceedings Volume 9750, 975005 (2016) https://doi.org/10.1117/12.2212135

KEYWORDS: Waveguides, Polymer multimode waveguides, Optical interconnects, Optical properties, Ultraviolet radiation

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Using the Mosquito method, we fabricate low-loss multimode polymer optical waveguides with graded-index (GI) perfect circular cores for the applications to on-board optical interconnection. We already developed the Mosquito method utilizing a microdispenser, as a fabrication technique for GI circular core polymer waveguides. In the Mosquito method, a liquid-state core monomer is dispensed from a syringe needle into a liquid-state cladding monomer while the needle horizontally scans. Originally we used siloxane based monomers. In this paper, novel organic-inorganic hybrid materials (SUNCONNECT®) are selected to confirm the applicability of wide-range polymers to the Mosquito method. Here, a dip is observed on the upper perimeter of the obtained core cross-sections particularly when using a straight needle. Such a core-shape deformation increases the coupling loss with circular-core optical fibers. So, the flow of core and cladding monomers while dispensing the core with the needle scan is visually observed. It is confirmed that the edge of the straight needle chips off the upper perimeter of the core when the core monomer is dispensed, leading to the dip. Therefore, the straight needle is replaced for a curved one to change the dispensing direction for eliminating the dip. It is experimentally found that an almost circular core (50-μm diameter with 1.09 vertical to horizontal ratio of diameter) is formed when a curved needle is used. Finally, we successfully demonstrate a 1.73-dB lower loss in a 5-cm long waveguide compared to the one having the core with a dip.

Proceedings Article | 7 December 2013 Paper

Governing equations for electro-conjugate fluid flow

K. Hosoda, K. Takemura, K. Fukagata, S. Yokota, K. Edamura

Proceedings Volume 8923, 89234Z (2013) https://doi.org/10.1117/12.2033791

KEYWORDS: Electrodes, Ions, Dielectrics, Numerical simulations, Liquids, Particles, Diffusion, Protactinium, Computer simulations, Electrostriction

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An electro-conjugation fluid (ECF) is a kind of dielectric liquid, which generates a powerful flow when high DC voltage is applied with tiny electrodes. This study deals with the derivation of the governing equations for electro-conjugate fluid flow based on the Korteweg-Helmholtz (KH) equation which represents the force in dielectric liquid subjected to high DC voltage. The governing equations consist of the Gauss's law, charge conservation with charge recombination, the KH equation, the continuity equation and the incompressible Navier-Stokes equations. The KH equation consists of coulomb force, dielectric constant gradient force and electrostriction force. The governing equation gives the distribution of electric field, charge density and flow velocity. In this study, direct numerical simulation (DNS) is used in order to get these distribution at arbitrary time. Successive over-relaxation (SOR) method is used in analyzing Gauss's law and constrained interpolation pseudo-particle (CIP) method is used in analyzing charge conservation with charge recombination. The third order Runge-Kutta method and conservative second-order-accurate finite difference method is used in analyzing the Navier-Stokes equations with the KH equation. This study also deals with the measurement of ECF ow generated with a symmetrical pole electrodes pair which are made of 0.3 mm diameter piano wire. Working fluid is FF-1EHA2 which is an ECF family. The flow is observed from the both electrodes, i.e., the flow collides in between the electrodes. The governing equation successfully calculates mean flow velocity in between the collector pole electrode and the colliding region by the numerical simulation.

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